Catalytic, hollow, refractory spheres, conversions with them

ABSTRACT

Improved, heterogeneous, refractory catalysts are in the form of gas-impervious, hollow, thin-walled spheres (10) suitable formed of a shell (12) of refractory such as alumina having a cavity (14) containing a gas at a pressure greater than atmospheric pressure. The wall material may be itself catalytic or a catalytically active material coated onto the sphere as a layer (16), suitably platinum or iron, which may be further coated with a layer (18) of activator or promoter. The density of the spheres (30) can be uniformly controlled to a preselected value within ±10 percent of the density of the fluid reactant such that the spheres either remain suspended or slowly fall or rise through the liquid reactant.

DESCRIPTION Origin of the Invention

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 83-568 (72 Stat435; 42 USC 2457).

Cross-Reference to Related Application

This is a division of application Ser. No. 841,062, filed Mar. 18, 1986,now U.S. Pat. No. 4,701,436, which in turn is a continuation-in-part ofapplication Ser. No. 602,901, filed Apr. 23, 1984, now U.S. Pat. No.4,576,926.

BACKGROUND OF THE INVENTION

The present invention relates to heterogeneous catalysts and moreparticularly, this invention relates to hollow, gas-impervious,catalytic spheres having a preselected density for the controlled anduniform conversion of catalytically reactive fluid materials.

Statement of the Prior Art

The relative merits of homogeneous and heterogeneous catalysts are wellknown. Homogeneous catalysts have better defined active sites, usuallyhave all of the metal available for catalysts, and offer steric andelectronic environments of the metal atom that can, at least inprinciple, be varied at will. The major disadvantage of homogeneouscatalysts is the need to separate them from reaction products withoutloss of their valuable metal content. This step can be both complex andexpensive. Other disadvantages are that these catalysts are relativelyeasily deactivated through aggregation or by poisonous by-products or atextreme temperatures. Also, corrosion of reactors by metal complexes iscommon.

Heterogeneous catalytic processes are of great industrial importance.Annually, 10,000 metric tons of ammonia are produced by directcombination of nitrogen and hydrogen gases at 400° C. and high pressureover iron catalysts promoted by several percent K₂ O and A1₂ O₃. Largevolumes of sulfuric acid and methanol are also produced by heterogeneouscatalysis. About 70 percent of all petrochemicals and refined petroleumproducts are produced by heterogeneous catalytic processes.Hydrogenation in presence of noble metals such as platinum or palladiumor transition metals such as nickel or cobalt can be used to convertcarbon monoxide to many different products such as ketones or alcohols,to convert oelfins to alkanes, benezene to cyclohexane or nitro groupsto amine groups. Transition metal catalysts also show activity for awide variety of industrially important reactions such as isomerization,hydroformylation, carbonylation, etc. These catalysts can be used toconvert pyrolysis coal gases into synthetic fuels such as oxo alcohols.

Heterogeneous catalysts have been developed in which the homogeneouscatalyst is either impregnated onto or chemically bonded to a solidsupport. Reaction rate is also dependent on surface area, and manycatalysts are provided in finely divided form such as fine powders ofplatinum prepared by reduction of the oxide. Catalysts are also preparedby impregnating the active catalyst onto high area supports, forexample, platinum deposited onto alumina particles having surface areasof the order of 100 square meters per gram. Heterogeneous catalysts havebeen prepared by coating the catalyst onto a hollow, porous support.

Baer, et al. (U.S. Pat. No. 3,347,798) prepare hollow, catalytic beadshaving a diameter greater than 90 microns for a fluidized bed process.The beads are gas-permeable so that reactants can diffuse into the coreand react with the inner wall. The beads are formed by spraying ahydrogel such as silicic acid or alumina containing a vaporizableexpanding agent through a nozzle into a tower and impinging the streamwith a gas heated to 300°-700° C.. Pilch, et al. (U.S. Pat. No.3,538,018) disclose an improvement over Baer, et al., in which compactcatalyst particles are added to the hollow spheres to form a mixturehaving a controlled density. Ao (U.S. Pat. No. 3,798,176) manufacturescontrolled-density, catalyst pellets having a vacant or a dense center.Ao forms a vacant center pellet; a thin polymeric shell is coated with acarrier and catalyst particles. During calcination, the core burns awayand the particles sinter and consolidate into a gas-permeable shell. Inthe dense center pellet, the core is made by pelletizing a ceramic.Martin (U.S. Pat. No. 3,978,269) forms porous pellets for automotiveexhaust reactors by coating liquid droplets with a powder mixture ofceramic and binder and then firing to form a porous, breatheable, hollowpellet. Watson, et al. (U.S. Pat. No. 4,039,480) also form an automotivecatalyst by coating a dry core with a dispersion of ceramic and firingto form a product having a bulk density below 50 lbs/ft³. Barnes, Jr.(U.S. Pat. No. 4,292,206) incorporates tiny, hollow glass spheres in thealumina powder mix as a lightweight filler to reduce weight of theresultant catalyst beads.

These porous catalytic particles are not uniformly dispersed in thefluid reaction media. They require separation of the reactant andproduct from the catalyst. The ceramic or polymer supported catalystparticles tend to crack, corrode or decay, which clogs the catalyst bedand requires shutting the reactor down to replace the bed and incurs theexpense of replacing the catalyst. Irregularly-shaped catalysts are notan optimum shape for catalytic reaction kinetics. The metal catalyst areheavy and would sink to the bottom of the reactor unless the reactionmedia is stirred or the catalyst circulated through it.

STATEMENT OF THE INVENTION

Improved heterogeneous catalysts are provided in accordance with thepresent invention. The catalysts of the invention have a uniform,controlled density that can be preselected such that the spheresdisperse uniformly throughout a fluid reaction medium or rise or fallthrough the medium at a preselected rate. The catalyst particles of theinvention are very strong and physically tough and will not crack, chipor abrade. The particles can be used and reused for numerous runs beforerequiring any regeneration or reprocessing. The catalyst of theinvention provides a large surface area for optimum contact of reagentand catalyst while assuring unobstructed flow of reactants through thebed of catalyst.

The catalyst of the invention automatically distributes through thefluid reaction medium or can flow through the body of fluid reactionmedium without requiring shaking, stirring or pumping. The catalystparticles are readily prepared in large volume and uniform shape at lowcost by the process of the invention.

The improved catalyst provided by the present invention is in the formof gas-impervious, hollow, thin-walled spheres. The wall material mayitself be catalytic or the catalyst can take the form of a coating ontothe wall material. Additional layers of activators or promoters can becoated onto the sphere or the promoters or activators can be mixed intothe layer of catalyst. The density of the spheres can be accuratelycontrolled by controlling the internal gas pressure and/or wallthickness of the spheres to form a uniform batch of spheres in which theweight of the spheres varies less than +5 percent.

Spheres that are from 0.5 to 10 percent by weight lighter than thereaction media will slowly rise at a controlled rate through thereaction media. Spheres that have substantially the same density (+0.01to 0.5 percent by weight) as the reaction media will remain uniformlydispersed therein and spheres that are from 0.5 to 10 percent heavierthan the reaction media will slowly fall through the media. The spherescan be produced over a fairly large range in diameter, such as from 0.20to 5.0 millimeters and still provide sufficient surface area for thecatalytic reaction to proceed at an economic rate.

The spherical catalysts are readily dispersed with a minimal amount ofenergy and can reject exothermic heat to the surrounding reaction media.The catalyst of the invention is easy to handle and readily separatesfrom the reaction media for cleaning, reprocessing, regeneration orrecirculation. There is no problem with packed beds or with fluid flowsince the catalyst spheres maintain a uniform dispersion with separationbetween adjacent spheres. The catalysts of the invention provide optimumutilization of expensive catalyst materials since the catalyst materialsare provided on the surface. The inner, inert core of the particles isfilled with inert gas. The catalysts of the invention are applicable forall prior heterogeneous reactions such as hydrogenation, polymerizationor oligomerization, isomerization, etc.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a catalyst sphere according to theinvention;

FIG. 2 is a schematic view of a batch reactor containing a uniformdispersion of catalyst spheres;

FIG. 3 is a schematic view of a fluidized bed reactor containing a bedof catalyst spheres;

FIG. 4 is a schematic view of a reactor with a falling column ofcatalyst spheres;

FIG. 5 is a schematic view of a reactor with a rising column of catalystspheres; and

FIG. 6 is a schematic view of a continuous flow reactor with continuouscirculation of catalyst and continuous introduction of reactant andremoval of reaction product.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, the catalyst of the invention is in the form ofa hollow sphere 10 having a gas-impervious shell 12 formed of a metal orrefractory material. The hollow, interior cavity 14 contains a gas suchas air under pressure. The pressure of the gas and the thickness andweight of the shell 12 and any promoter or initiator layers thereon areselected to provide a predetermined density in relation to the densityof the fluid reaction media. The shell 12 may be formed of acatalytically active metal such as platinum or aluminum or anoncatalytically active refractory support such a alumina on which iscoated a layer 16 of catalyst, e.g., platinum or iron. The catalystlayer 16 may be from 0.1 μm to 0.1 mm in thickness. The catalyst layer16 may contain from 1 to 25 percent by weight of a promoter, or theinitiator may be coated onto the surface of the catalyst layer. Thelayers 16, 18 may be formed by deposition of the compounds from thevapor or liquid phase such as by thermal decomposition of metalcarbonyls and the like.

Refractory materials that withstand high temperature of 2000° F. to5000° F. or more without melting or decomposing fluid use in hightemperature catalytic reactions. The refractory material can be a metalor alloy such as tungsten or a metal compound such as an oxide, nitride,carbide, boride or silicide, silicate, aluminate or mixtures thereof.The oxide can be a magnesia, beryllia, silica or alumina or mixturesthereof. The hollow, refractory catalytic sphere may have inherentcatalytic properties or can be modified to have catalytic properties byadding 0.1 to 25 percent by weight of a catalytic metal such as cobalt,molybdenum, nickel, chromium, zinc, iron, copper, tungsten, silver, tin,vanadium, platinum or palladium to the refractory shell or by coatingthe metal onto the surface of the refractory sphere. Catalysts,particularly useful as oxidizing catalysts for treating automobileexhaust are formed by coating platinum onto alumina or aluminosilicatespheres.

Gas-filled, spherical, metal or refractory shells, that aredimensionally precise, smooth and of high strength can be produced by amethod based on the hydrodynamic instability of an annular jet of moltenmetal as disclosed in U.S. Pat. No. 4,344,787, the disclosure of whichis expressly incorporated herein by reference. The basis of the methodrests upon the phenomenon of instability and breakup of a jet flow ofliquid as it issues into a gaseous medium at rest. In the embodimentemployed herein, a coaxial flow of fill gas is provided at the core of acircular jet by means of a thin-wall tube. When the axial velocities ofthe jet liquid and of the central gas are adjusted to fall withincertain ranges, the jet exhibits an instability which generateslarge-amplitude axisymmetric oscillations. These culminate in a rapidpinchoff of the jet and in the formation of a liquid shells which can bedescribed as thick-wall bubbles. A remarkable feature of the instabilityis that it is more powerful by far than the familiar Kayleighinstability of a nonhollow jet.

The oscillation growth is so rapid that the nonlinear motion regime isattained within three or four jet diameters, and pinchoff ensuresquickly. The motion is highly deterministic; although the action occursspontaneously and without external stimulus, a frequency stability andcorresponding uniformity in shell mass exceeding one part in 10³ isreadily attained. As each shell in turn parts from its neighbors, itundergoes a ringing oscillation which has the beneficial effect ofpromoting a centering of captured gas.

The dimensions of shells produced by nozzles of this type may be variedover wide limits. In accordance with the physical process of shellformation, the diameter of the product will be approximately twice thatof the jet orifice, whereas the relative wall thickness is not so simplydetermined. That quantity is not only a function of the aspect ratio ofthe annular passageway, but is simultaneously a function of the volumeflow rate of the fill gas. An increase in the gas flow rate at fixedliquid rate results in an increase in the bubble formation frequency andin a concommitant decrease in wall thickness.

Tin and aluminum shells ranging in diameter from 750-2000 μm and wallthicknesses of about 25 μm have been formed in quantity. Here, the metaljet issued into ambient air. Examination of specimens was made by meansof scanning electron micrography (SEM). For shells at the upper sizelimit, it was found that the specimens were spherical to within aboutone percent except near two diametrically opposing points from which thejet pinchoff had occurred.

Metallic shells have been produced by the jet instability method atrates up to a few thousand per second. The shells exhibit excellentuniformity in size, good sphericity over most of the surface and fairconcentricity. The spheres have excellent surface quality and hightensile strength. The shells are then further coated with catalyst andpromoter layers. As a specific example of practice, aluminum spheresabout 2 mm in diameter, with a wall thickness of about 25 μm and aninternal air pressure of about 100 psi could be coated with a 0.1 μmthick layer of platinum by reduction of precursor oxide powder.Palladium or nickel could also be coated onto the alluminum shell bythis technique. These catalysts could be utilized for hydrogenationreactions. When the aluminum shell is coated with iron, the spheres canbe utilized to produce ammonia by combining N₂ and H₂ gases at 400° C.and several hundred atmospheres of pressure. Further coating the ironlayer with a layer of Na₂ O or K₂ O results in a synthesis gasconversion catalyst which converts CO and H₂ gases into produce gascontaining CH₄, C₂ H₆, C₃ H₈, C₉ H₁₀, other alkanes, olefins, alcohols,aldehydes and acids.

The ability to adjust the buoyancy of the uniformly-sized spheres makepossible reaction processes in which the body of reaction medium isstationary and the catalyst moves through the reaction medium at acontrolled rate, or is uniformly suspended therein. A process utilizingcatalytic, hollow spheres 30 having a density differing from the liquidmedia 32 by +0.1 to 0.5 weight percent is illustrated in FIG. 2. As thecatalyst particles 30 are fed from the hopper 34 into the reactor 36,they will deploy to form a uniform suspension 37 within the body 36 ofliquid reaction medium. This reactor can be operated as a batch reactorin which reactants are introduced through inlet 38 and are removed atthe end of the run through outlet 40. The catalyst can be separated bymeans of a screen or filter 42 and recycled to the hopper 34.

Continuous flow processes can be operated by flowing reaction mediumthrough a bed of catalyst restrained between porous barriers such asscreens. The catalyst spheres may tend to pack against the upstreamscreen unless they are allowed to expand as in a fluidized bed reactoras shown in FIG. 3. The catalyst particles 30 are placed in the reactor31 downstream of perforated plate 42. The gaseous or liquid fluidreactant is introduced through the inlet 44 placed upstream of theplate. The particles 30 expand by the action of the flowing stream toform a fluidized bed 46. A further screen or perforated plate 48 may beplaced towards the top of the reactor to prevent any catalyst particlesfrom being carried out the outlet 50 with the reaction products.

A batch reactor with an autogeneously moving catalyst suspension isillustrated in FIG. 4. In this process, the catalyst particles 52 are ofuniform size and density and have a density preselected to a value from0.5 to 10 percent heavier than that of the liquid reaction media 54within the reactor 56 so that the transmit time of the particles withinthe reactor provides a desired degree of conversion of the reactants. Asthe catalyst spheres 52 deploy into the reaction media 54 from thehopper 58, they will form a uniform suspension which slowly falls to thebottom of the vessel at a controlled rate. Reactants can beintermittently or continuously fed to the reactor from the inlet 60 andreaction product can be intermittently or continuously removed throughoutlet 62 containing a liquid-solid separator 64 to remove catalystparticles 52 for recycle to the hopper 58.

FIG. 5 illustrates a batch or continuous process utilizing catalystparticles 70 which are lighter than the reaction media by 0.5 to 10percent by weight and have a density preselected to provide a desiredrate of travel of the particles through the column 72 of reactant. Thespherical catalyst particles 70 are fed from a supply vessel 74 into thebottom of the reactor 76 and slowly rise as a suspension through thecolumn 72 of reaction medium. The catalyst particles can be removed by askimmer 78 and recycled through line 80 up to the supply vessel 74. Thereaction product can be recovered through an outlet 82 or by overflowintermittently or continuously. Similarly, reactant material can beintroduced through the inlet 84 continuously or intermittently.

In the embodiment shown in FIG. 6, the catalyst suspension and reactioncolumn move continuously under counter-current flow conditions. Thisprocess also illustrates use of two different catalysts for a two-stageprocess. The reactor 80 includes two circulation loops 83, 85. In thefirst loop 83, a first reactant, R¹, is fed into the inlet 86 from thesupply tank 88 and pump 90 and forms a rising column 92 of reactionmedium. First stage product can be removed through outlet 94 andrecycled through line 96 to the inlet side of the pump 90 when thethree-way valve 98 is turned toward the recycle line 100.

First stage catalyst 91 is fed into the inlet 102 of loop 85 from hopper104, rises through the column 92 of reaction medium and is removedthrough outlet 106. The first stage catalyst is recycled through line108 until the reaction is complete. Valve 110 is turned toward line 112and all the first stage catalyst is returned to the hopper 104. Valve114 is opened to feed second stage catalyst 116 into the loop 85 andreaction column 92. Additional reactant, R², may now be fed from tank116 into inlet 86 by opening valve 118.

The three-way valve 98 is turned toward recycle. The second stagecatalyst is cycled through the column 92 until the reaction is complete.The three-way valveis then turned toward vessel 120 and the second stagereaction produce is recovered.

It is to be understood that only preferred embodiments of the inventionhave been described and that numerous substitutions, modifications, andalterations are permissible without departing from the spirit and scopeof the invention as defined in the following claims.

We claim:
 1. A reaction medium media comprising a liquid reactantcontaining a suspension of hollow, gas impervious catalyst spheresformed of a refractory compound shell having a catalytic active surfaceand containing a gas at a pressure greater than atmospheric, saidspheres having a preselected density in the range of +10 percent of thedensity of the liquid reaction medium.
 2. A reaction media according toclaim 1 in which the spheres have a density within +0.5 percent of thedensity of the liquid reaction medium and form a uniform stationarysuspension therein.
 3. A reaction media according to claim 1 in whichthe spheres have a density less than the density of the liquid reactionmedium by 0.5 to 10 percent and form a suspension that slowly risestherethrough.
 4. A reaction media according to claim 1 in which thespheres have a density greater than the density of the fluid reactionmedium by 0.5 to 10 percent and form a suspension that slowly fallstherethrough.
 5. A reaction media according to claim 1 in which thediameter of the sphere is from 0.20 mm to 5.0 mm.
 6. A reaction mediaaccording to claim 5 in which the shell of the sphere consistsessentially of an oxide, nitride, carbide, silicide, boride, silicate,aluminate or a mixture thereof.
 7. A reaction media according to claim 1in which the catalytically active surface comprises a catalyst layerdeposited on the shell.
 8. A reaction media according to claim 1 inwhich the catalytically active surface is formed by catalytically activematerials present in the shell.
 9. A reaction media according to claim 7in which the catalyst layer includes a catalytically active metal.
 10. Areaction media according to claim 9 in which the catalytically activemetal is selected from transition metals and noble metals and thecatalyst layer includes an activator and/or promotor.
 11. A reactionmedia according to claim 1 in which the gas is air.
 12. A reaction mediaaccording to claim 1 in which the density of the spheres does not varymore than +0.5 percent by weight.
 13. A method of catalyticallyconverting a reactant into a product comprising the steps of:forming auniform suspension of hollow, gas-impervious catalytic refractorycompound spheres within a body of; catalytically reacting the reactantto form a reaction product; and separating the catalyst spheres from thereaction product.
 14. A method according to claim 13 in which thespheres have a uniform diameter and a preselected density within +10percent of the density of the fluid liquid reaction medium.
 15. A methodaccording to claim 14 inw hich the spheres have a density within +0.5percent of the density of the fluid liquid reaction medium and form auniform stationary suspension therein.
 16. A method according to claim14 in which the spheres have a density less than the density of thefluid liquid reaction medium by 0.5 to 10 percent and form a suspensionthat slowly rises therethrough.
 17. A method according to claim 14 inwhich the spheres have a density greater than the density of the fluidliquid reaction medium by 0.5 to 10 pecent and form a suspension thatslowly falls therethrough.
 18. A method according to claim 13 in whichthe diameter of the spheres is from 0.20 mm to 5.0 mm.
 19. A methodaccording to claim 13 in which the refractory compound is selected fromoxides, nitrides, carbides, silicides, borides, silicates, aluminates ormixtures thereof.